Federal prosecutors issued subpoenas to associates of President Donald Trump’s former national security adviser Michael Flynn, CNN reported on Tuesday.

Citing unnamed sources familiar with the matter, CNN reported that U.S. Attorney Dana Boente’s office is seeking business records from the period after Flynn was ousted as director of the Defense Intelligence Agency in 2014.

The subpoenas represent the first sign of a significant escalation of activity in the FBI’s broader investigation begun last July into possible ties between Trump campaign associates and Russia.

The subpoenas issued in recent weeks by the US Attorney’s Office in Alexandria, Virginia, were received by associates who worked with Flynn on contracts after he was forced out as director of the Defense Intelligence Agency in 2014, according to the people familiar with the investigation.